1. Technical Field
The present invention relates generally to thermoacoustic systems, and more particularly, to a mechanical resonator and method for thermoacoustic systems.
2. Related Art
Thermoacoustic systems may serve many purposes in modem society including energy conversion. For instance, thermoacoustic engines convert thermal power to mechanical power. These can be combined with generators that convert mechanical power to electrical. Thermoacoustic systems driven by motors convert electrical, pneumatic or hydraulic power to mechanical and then to thermal output (cooling or heating). All of these devices depend on machinery to accomplish the conversion, and all have limits in cost, efficiency, and size, which make one type or another well or ill suited to particular applications.
Thermoacoustic devices such as those described in U.S. Pat. Nos. 4,114,380 and 4,355,517 to Ceperly and 4,398,398 and 4,489,553 to Wheatley, provide rugged, simple and low-cost conversion of heat energy to mechanical energy in the form of oscillating acoustic pressure and volume in a contained gas, or vice versa. These devices can provide engines or heat pump/coolers. The primary components of these devices are an elongate housing containing a compressible fluid, a warmer heat exchanger in thermal communication with an external reservoir near the warmer temperature, a cooler heat exchanger in thermal communication with a reservoir at or near that cooler temperature, and a thermodynamic medium in the form of either the fluid itself or an element such as a xe2x80x98stackxe2x80x99 or regenerator between the heat exchangers. The principles of operation of stacks are explained more fully in U.S. Pat. No. 4,489,553, which is hereby incorporated by reference. A device using a regenerator instead of a stack and including an additional fluid path (having acoustic inertance, capacitance, resistance or a combination thereof) creates a Stirling-like cycle in the compressible fluid particles near the regenerator. See, for instance, U.S. Pat. No. 4,355,517 to Ceperly. The above-described devices are commonly identified as xe2x80x98standing wavexe2x80x99 and xe2x80x98travelling wavexe2x80x99 types, respectively. The operation of these devices requires a resonant compressible fluid (gas) circuit to define and sustain the oscillations in the compressible fluid.
Unfortunately, creation of this resonant circuit requires a long, enclosed structure or housing, akin to an organ pipe, in which the fluid is contained. The length of the housing and the physical properties of the compressible fluid determine the operating frequency. For commonly-preferred gases (e.g., air, helium), the resulting length is too great for many uses.
A masters thesis by Larry A. Grant, entitled xe2x80x9cInvestigation of the Physical Characteristics of a Mass Element Resonator,xe2x80x9d dated 1992 (NTIA ADA2521792, originally from the Naval Postgraduate School at Monterrey, Calif.) discloses a bellows (having mass and stiffness) in lieu of a central part of a thermoacoustic resonator to xe2x80x9creduce those acoustic losses that are a parasitic load on the cold end of the refrigerator, as well as make the resonator more compact.xe2x80x9d While Grant introduces the concept of mechanical equivalence, the bellows structure disclosed has been found unworkable for everyday thermoacoustic devices. In particular, Grant""s studies related to a system that operates at a very high frequency similar to a piezo-electric system, while many thermodynamic devices suitable for general applications (e.g., those driven by 60 Hz grid electricity) operate at lower frequencies similar to a loudspeaker system. For these lower frequencies, practical systems require higher stroke and pressure amplitude than can be reliably sustained by a bellows as Grant disclosed. Uncontrolled secondary motions arise in the bellows and the material of the bellows succumbs to fatigue. Accordingly, Grant""s system does not translate to common thermoacoustic devices. No other structure was suggested by Grant.
A PCT application to DeBlok, WO 99/20957, discloses a traveling wave thermoacoustic system having a membrane or bellows construction that provides a mass-spring-system. Unfortunately, a membrane or bellows construction has been found unstable and, therefore, is inadequate to provide meaningful shortening of the gas resonator length.
In view of the foregoing, there is a need in the art for a device to shorten the length of housings in thermoacoustic devices so broader applications can be attained. It would also be advantageous if the device incorporated mechanisms for attaining energy conversion such as a transducer.
A first aspect of the invention is directed to a mechanical resonator for a thermoacoustic device having a compressible fluid contained within a housing having a pair of heat exchangers and a thermodynamic medium therebetween, the resonator comprising: a member for mimicking dynamic conditions at a position of the housing; and a linear suspension element suspending the member in the housing.
A second aspect of the invention is directed to a thermoacoustic system comprising: a housing enclosing a compressible fluid capable of supporting an acoustical wave; a first heat exchanger; a second heat exchanger; a thermodynamic medium interposed between the heat exchangers for sustaining a temperature gradient in the compressible fluid between the heat exchangers; and a mechanical resonator mounted in the housing adjacent the heat exchangers, the mechanical resonator including: a member mounted for reciprocation along a direction of fluid oscillation and to form a substantial barrier to passage of the compressible fluid, and a linear suspension element for suspending the member during reciprocation, the suspension element coupled to the housing.
A third aspect of the invention is directed to a method for shortening a thermoacoustic device having a housing for containing a compressible fluid and thermodynamically active components therein that operate at a known frequency and a known temperature, the method comprising the steps of: determining dynamic conditions at a position within the housing; and replacing at least a portion of the fluid and housing adjacent the position by suspending a mechanical resonator having a member that matches the dynamic conditions at the position within the housing.
A fourth aspect of the invention is directed to a thermoacoustic system comprising: a) a housing enclosing a compressible fluid capable of supporting an acoustical wave; b) a standing wave thermoacoustic subsystem including: a first heat exchanger, a second heat exchanger, wherein the second heat exchanger is cooler than the first heat exchanger, and a thermodynamic medium interposed between the heat exchangers for sustaining a temperature gradient in the compressible fluid between the heat exchangers; c) a mechanical resonator mounted for reciprocation along a direction of fluid oscillation and to form a substantial barrier to passage of the compressible fluid; and d) a transducer coupled to the mechanical resonator.
A fifth aspect of the invention is directed to a mechanical resonator for a thermoacoustic device having a compressible fluid contained within a housing, the housing having a pair of heat exchangers and a thermodynamic medium therebetween, the resonator comprising: a member adjacent a cooler one of the heat exchangers; and a thermal insulation on the member.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention.